I would either go much wider (I love the Tokina 11-16 f2.8 ) or go for a more standard lens, like a canon 28mm f2.8 or Canon 35mm f2.0.
He already has the Voigtlander 40mm f/2, so getting the 35 f/2 won't do him any good.
On the other end, for shooting casual stuff of friends and family, the Tokina is way too wide. It's a great lens if you know how to use it. But, of the lenses I own, it'd be the last one I would pick to shoot people, because of the distortion. Even at 16mm you'll find it annoying.
Personally I wouldn't spend that much on the 24mm f/1.4 just to shoot friends, but, if I had the money it'd be a nice lens to own. I think the IS primes are way overpriced, and you'd get more bang for your buck going another way. I'd either get one of the older non-IS primes and a decent stabilizer, or look for one of the IS enabled zooms (Canon 17-55, Sigma 17-50, Tamron 24-70, etc). You'd also gain a constant aperture, which is important for video work, and you'll gain a stop or two from the 15-85 you currently own.